The present invention relates to devices in which current signals are amplified and converted into voltage signals for processing. Such devices have a wide application range and may for example be found in systems where sensors delivering a current are used to monitor processes and/or to measure physical parameters of an environment.
Various sensors delivering a current find applications in a measurement of physical parameters. Such a sensor may for example be a magnetical head which comprises a gap and delivers a current when moved in the presence of a magnetical field. This current is proportional to the magnetic field. Another example for such a sensor is a photodetector which delivers a current proportional to a number of photons detected. Often the currents delivered by such sensors have comparatively small values i.e. the current signals may easily be subjected to interference from electromagnetic radiation emitted by sources nearby the sensor. The interference from this electromagnetic radiation may lead to erroneous measurements because the currents delivered by the sensors are no longer predictable functions of the physical parameters to measure. In order to minimize the negative impact of interference on the current signals it is known to amplify the current signals as near to the sensor as possible and obtain amplified signals for which the interference with the electromagnetic radiation is less critical.
In known Optical disk players an optical disk is scanned using a light beam and light reflected by the optical disk is measured using a plurality of photodetectors. The photodetectors are mounted on a movable optical pickup which is moved relatively to the disk in order to position the light beam relatively to the optical disk. The photodetectors deliver current signals having values in the range of for example a few μA. These current signals are easily disturbed through interference with electromagnetic fields generated by positioning coils which are also comprised in the movable optical pickup. Therefore the current signals are amplified with amplifiers mounted on the same movable optical pickup near to the photodetectors. The amplified signals are voltages with values in the range of a few mV. The amplified signals are transported to further amplifiers where they are amplified to voltage signals which have values in the range of a few V. The further amplifiers are distanced from the movable optical pickup and connected to the amplifiers on the movable optical pickup using flexible electrical conductors. The voltage signals are thus representative of the current signals delivered by the photodetectors and may for example be used for further digital processing i.e. decoding of the signals read from the disk. It is of great importance that the voltages signals obtained be a reliable representation of the current signals and in this sense to minimize the number of errors which might occur at the amplification and transportation of the current and amplified signals.
During transportation from the amplifier to the further amplifiers the amplified signals may mutually influence each other because of crosstalk between the flexible electrical conductors. This is due to the fact that the amplified signals are voltages and this causes even more errors in the voltage signals at the output of the further amplifiers.
Most known optical disk players make use of error correction methods and may correct a limited numbers of errors in the decoded signals read from the disk. As has been said previously, a first source of errors may be due to amplification and transportation of the current and amplified signals, but also to crosstalk between flexible electrical conductors. A second source of errors may be disturbances on the disk being read e.g. scratches, fingerprints or aging related degradation of layers used to store the signal on the disk. If all errors resulting from the addition of the first and second source of errors need to be corrected then the value of the addition must not be greater than the limited number which may be handled by the error correction method. While the second source of errors most frequently and inevitably affects optical disks, the first source depends on characteristics of the optical disk player. These characteristics may be influenced on by a manufacturer of the optical disk player and eventually determine a quality of the latter player.
The amplifiers and further amplifiers mentioned above are respectively current to voltage (I:U) and voltage to voltage amplifiers (U:U). Each of these amplifiers and further amplifiers, e.g. operational amplifiers, may use a predetermined voltage UD to operate, i.e. UD is used as a reference voltage to output respectively the amplified signals and voltage signals at a determined voltage value. UD is determined relatively to a ground potential UG (which should be UG=0 V) which is common to all part of the optical disk player and to which each amplifier is connected. The amplifiers mounted on the movable optical pickup are connected to ground via a flexible electrical conductor which has a determined resistance value. The consequence of the determined resistance value is that the amplifier's ground potential isn't the same as the further amplifier's ground potential and the reference voltage UD which should be the same for both amplifiers and further amplifiers varies. Eventually the voltage signals at the output of the further amplifiers may have values which are erroneous due to an altered reference voltage applied to the further amplifiers. As mentioned above the voltage signals may be used for further digital processing and a change in their value, i.e. an erroneous value, may cause loss of bits. This loss of bits is part of the previously discussed first source of errors.
In a similar way as for optical disk players the processing of current signals providing from sensors in other devices (e.g. magnetic tape players, speed or process monitoring in cars or industrial machines) may produce erroneous values because the current signals are amplified more than once with amplifiers which are distant from each other and have to share a common ground potential which may vary from one amplifier to the other.
It is one aim of the present invention to overcome the problem of erroneous voltage signals resulting from amplification and transportation of current signals providing from sensors as described above.
It is another aim of the present invention to provide for optical disk players which have a reduced number of errors from the first source of errors.